ACT SSC Biology — Unit 3
Speciation — Flashcards & Quiz
Speciation is the formation of new biological species from existing ones through the accumulation of genetic differences and the development of reproductive isolation. ACT SSC Biology Year 12 Unit 3 distinguishes allopatric (geographic isolation) from sympatric (no geographic isolation) speciation and asks you to explain prezygotic and postzygotic isolation mechanisms.
Key Points
- Speciation requires reproductive isolation: gene flow between populations must stop (or nearly so) to allow divergence.
- Allopatric speciation: a physical barrier (river, mountain, ocean) separates a population into two; each diverges independently.
- Sympatric speciation: new species form without geographic isolation — e.g. through polyploidy (common in plants) or ecological niche partitioning.
- Prezygotic isolation prevents mating or fertilisation: temporal (different breeding seasons), behavioural (different mating rituals), mechanical (incompatible anatomy), gametic (sperm and egg incompatible).
- Postzygotic isolation prevents viable/fertile offspring: hybrid inviability, hybrid sterility (e.g. mules), hybrid breakdown in later generations.
- Biological species concept: a species is a group of interbreeding populations reproductively isolated from other such groups.
Common Mistakes to Avoid
- Confusing allopatric (geographic barrier) with sympatric (no geographic barrier) speciation.
- Mixing up prezygotic (before fertilisation) and postzygotic (after fertilisation) isolation.
- Claiming mules are a separate species — they are hybrid sterile individuals, not a species.
- Ignoring the role of genetic drift in small founder populations.
- Applying the biological species concept to asexual organisms, where it doesn't work.
Exam Strategy
BSSS Unit 3 speciation questions give you a scenario and ask you to identify the type and explain the mechanism. Method: (1) identify whether geographic isolation is present (allopatric vs sympatric), (2) name the specific isolation mechanisms (pre- or postzygotic), (3) explain how genetic divergence accumulates, (4) link to named case studies (e.g. Galapagos finches, cichlid fish in African lakes).
Sample Flashcards
Q1: What is speciation?
Speciation is the formation of new species. It occurs when populations become reproductively isolated and evolve independently through natural selection, genetic drift and mutation until they can no longer interbreed.
Q2: Distinguish allopatric from sympatric speciation.
Allopatric: a geographic barrier physically separates populations. Sympatric: new species arise within the same geographic area through polyploidy, habitat differentiation or behavioural isolation.
Q3: What is reproductive isolation?
Reproductive isolation prevents interbreeding between populations. Pre-zygotic barriers (habitat, temporal, behavioural, mechanical, gametic) prevent mating. Post-zygotic barriers (hybrid inviability, sterility) prevent viable offspring.
Sample Quiz Questions
Q1: Allopatric speciation requires a geographic barrier.
Answer: TRUE
A physical barrier separates populations, preventing gene flow and allowing independent evolution.
Q2: Reproductive isolation is not necessary for speciation.
Answer: FALSE
Reproductive isolation is essential — without it, gene flow prevents populations from diverging into separate species.
Revision Tip
Speciation mechanisms are classification-heavy — drill a Revizi deck with 5+ real-world examples (finches, cichlids, Drosophila, polyploid plants) assigning each to its speciation type.
Last updated: March 2026 · 3 flashcards · 2 quiz questions